Three-dimensional radiative transfer in clumped hot star winds

Abstract

Context. The true mass-loss rates from massive stars are important for many branches of astrophysics. For the correct modeling of the resonance lines, which are among the key diagnostics of stellar mass-loss, the stellar wind clumping has been found to be very important. To incorporate clumping into a radiative transfer calculation, three-dimensional (3D) models are required. Various properties of the clumps may have a strong impact on the resonance line formation and, therefore, on the determination of empirical mass-loss rates. Aims. We incorporate the 3D nature of the stellar wind clumping into radiative transfer calculations and investigate how different model parameters influence the resonance line formation. Methods. We develop a full 3D Monte Carlo radiative transfer code for inhomogeneous expanding stellar winds. The number density of clumps follows the mass conservation. For the first time, we use realistic 3D models that describe the dense as well as the tenuous wind components to model the formation of resonance lines in a clumped stellar wind. At the same time, we account for nonmonotonic velocity fields. Results. The 3D density and velocity wind inhomogeneities show that there is a very strong impact on the resonance line formation. The different parameters describing the clumping and the velocity field results in different line strengths and profiles. We present a set of representative models for various sets of model parameters and investigate how the resonance lines are affected. Our 3D models show that the line opacity is lower for a larger clump separation and shallower velocity gradients within the clumps. Conclusions. Our model demonstrates that to obtain empirically correct mass-loss rates from the UV resonance lines, the wind clumping and its 3D nature must be taken into account.

Mass loss from inhomogeneous hot star winds

“…3 compare line profiles computed using this vmSEI approach with profiles computed using the standard SEI model, for intermediate and strong lines with κ 0 = 1 and 100. The figure shows clearly the basic velocity-porosity effect, namely weaker line-profiles for a given line-strength parameter κ 0 , consistent with all previous work on the effects of optically thick clumps on UV wind lines (Oskinova et al 2007;Sundqvist et al 2010Sundqvist et al , 2011Šurlan et al 2012.…”

“…This excess is caused by photon trapping within the resonance zones and by increased back-scattering due to multiple such resonance zones (Lucy 1984;Puls et al 1993;Sundqvist et al 2010), which allows light to escape primarily when emitted on the red side of the line profile; such multi-scattering effects cannot be simulated within the simple effective opacity method developed in this paper, but does not affect the absorption line strength that is the primary focus here (and in general when using unsaturated resonance lines as diagnostic tools). Moreover, in particular the κ 0 = 100 line with a void inter-clump medium shows a prominent absorption-dip toward the blue edge of the profile (see also Sundqvist et al 2010;Šurlan et al 2012). In the stochastic wind models, overlapping clumps in velocity space and the finite extent of the line profile lead to an increase in f vor at high velocities, and so results in more efficient absorption in the outermost wind than in the accelerating parts of it.…”

“…In Papers I and II of this series (Sundqvist et al 2010(Sundqvist et al , 2011, we developed detailed multi-dimensional wind models to study the effects of vorosity on the formation of, in particular, the strong UV "P-Cygni" lines that are the classical trademarks of massive star winds. A key general result from these studies is that clumps indeed easily become optically thick in such UV lines, and that the associated additional leakage of photons leads to weaker line profiles than predicted by smooth or volume filling factor models (see also Oskinova et al 2007;Hillier 2008;Šurlan et al 2012. But constructing realistic ab-initio radiation-hydrodynamic wind simulations that account naturally for spatial and velocity-field porosity is an extremely challenging and time-consuming task.…”

The evolution of massive stars and their spectra

“…The effects of clumping on these and other spectral lines are well known and have been extensively discussed in the literature Bouret et al 2003Bouret et al , 2005Bouret et al , 2013Hillier et al 2003;Fullerton et al 2006;Martins et al 2007Martins et al , 2009Martins et al , 2012Oskinova et al 2007;Sundqvist et al 2010Sundqvist et al , 2011Šurlan et al 2012;Zsargó et al 2008).…”

Mass-loss rates from mid-infrared excesses in LMC and SMC O stars

“…To progress, a firm grasp on the underlying physical mechanisms which determine the wind structures is required. The state of affairs can be seen in recent literature where the values of observationally derived mass loss rates have swung back and forth by factors of 10 or more (Puls et al 2006, Massa et al 2003, Fullerton et al 2006, Sunqvist et al 2011, Surlan et al 2012). …”